1. Field of the Invention
This invention relates generally to compact, light weight printheads and, more particularly, to integral Organic Light Emitting Diode (OLED) printheads.
2. Background
Light emitting diodes (LED) have been used for exposing photosensitive materials such as photographic film or photosensitive paper or photocopying receptors. The light emitting diodes are usually arranged in a linear array or a number of linear arrays and means are provided for a relative displacement of the photosensitive materials in relation to the array. In this manner, the material is scanned past the array and an area is exposed thereby creating an image.
The light emitted from LEDs diverges quickly and thus reduces the exposing intensity and increases the exposing area. This can lead to a reduction in sharpness of the exposed image and to the possibility of undesired exposure of adjacent areas. The first of these problems is known as reduced pixel sharpness and the second is known as crosstalk. To avoid these difficulties, optical systems are utilized to transmit the light from the LEDs to the photosensitive material without significant divergence. While this approach results in an acceptable printing system, such systems have their size defined by the optical systems and therefore are not as compact as would be desired for a portable print system.
The light emitting diodes used in present printers (see for example, Shimizu et al., LED Arrays, Print Head, and Electrophotographic Printer, U.S. Pat. No. 6,064,418, May 16, 2000) emit radiation from the surface of a p-n junction (constitute edge emitters) and are typically mounted on a printed circuit board. These characteristics of the LEDs used in previous printers impose constraints on manufacturability and preclude their use in contact or quasi-contact printing. Other light source systems (LCDs for example) presently used in printers suffer from similar constraints that preclude their use in contact or quasi-contact printing. Innovative designs are needed to satisfy the need for compact printers.
Organic Light Emitting Diodes (OLED), which have been recently developed, (See, for example, the article by S. Forrest, P. Burrows, M. Thompson, “The Dawn of Organic Electronics”, IEEE Spectrum, Vol. 37, No, 8, pp. 29-34, August 2000) hold a promise of ease of fabrication and low cost and low power consumption. A recent publication (Y. Tsuruoka et al., “Application of Organic Electroluminescent Device to Color Print Head”, SID 2000 Digest, pp. 978-981), describes a print head utilizing OLEDs. The printhead described in this publication is comprised of discrete OLEDs, color filters and optical elements and therefore is not as compact as desired. Also, the presence of discrete optical elements requires considerations of alignment which have an impact on manufacturability and cost.
It is the primary object of this invention to provide an integral printhead which is compact and light weight and utilizes Organic Light Emitting Diodes (OLED). It is a further object of this invention to provide an integral printhead which avoids crosstalk while providing the necessary pixel sharpness and utilizes Organic Light Emitting Diodes (OLED). Other objects of this invention will become apparent hereinafter.